ESPN does not own the media rights to Clemson and FSU, the ACC does. The ACC schools did not sign their GoR with ESPN, they signed it with the ACC who then leased their rights to ESPN (and Raycom). If a school leaves, the only thing ESPN can do is change the contract as the terms of the agreement were changed (a school left).
If the rumored payout by FSU and Clemson to leave the ACC is correct ($300 million each broken up evenly over 10 years), that would more than cover what ESPN might take away. Could ESPN leverage the ACC by dropping the contract down to an unacceptable level? Sure, but Disney is in no financial position to fight anything right now.
Do I believe the rumor? Not really, it came from one of the CR Twitterati.